The purpose of this study is to compare an IV loading dose of Azathioprine with a placebo IV loading dose for the induction of remission in active Cohn's disease. Remission is defined as a CDAI score <150 and total steroid withdrawal by 8 weeks. Secondary objectives are: 1) To compare the two treatments for red blood cells 6-thioguanine nucleotide 9(TBC 6TGN) concentrations; 2) To correlate RBC 6TGN concentrations with efficacy and safety results; 3) To compare the two treatments for safety; 4) To compare the two treatments for quality of life endpoints. Randomized patients will be hospitalized and receive a continuous infusion of blinded drug for 36 hours. Patients will then be dismissed from the hospital and will receive open label therapy for 16 weeks. The active metabolites of Azathioprine (AZA), the RBC 6TGNs, have a long half-life and accumulate gradually over weeks to months in subjects being treated with AZA. Pretreatment with an IV loading dose of AZA (40 mg/kg/day) before beginning oral AZA 2.0 mg/kg/day in subjects with Cohn's disease will decrease the time required to reach TBC 6TGN steady state concentration and will decrease the time to clinicl response.